It is known that a laser apparatus which must provide a certain power level is subjected to considerable heating. These arrangements generally comprise a discharge tube when concerning a gas laser or a bar when concerning a solid medium laser. In principle the tube or the bar is supported by two flanges maintained at a constant distance with the purpose of assuring good stability for the laser. However, in being heated the tube or the bar is displaced in said flanges and arrangements must be made in order that the relative movement may take place freely whilst assuring perfect sealing of the elements which are displaced with respect to one another.
A construction which enables the displacement as mentioned is described in U.S. Pat. No. 3,705,999. Here it concerns a gas laser equipped with a discharge tube 55 arranged to slide freely in a flange 50 fixed to an annular collar 52 as is shown by FIG. 2 of said document. A bellows 53 intended to compensate variations of length due to temperature variations of the tube 55 is arranged between the collar 52 and a collar 54, itself fixed to tube 55. The bellows is brazed by each of its ends to the collars which it couples, assuring thus perfect sealing towards the exterior of the gas contained in the tube. Since here the matter concerns collars having the same coefficient of thermal expansion, there is no difficulty in choosing a bellows having likewise the same coefficient and brazed on either side to the collars as mentioned.
The gas laser described in U.S. Pat. No. 3,611,183 includes a sealed quartz tube in which there are arranged a central anode and two cathodes at the ends. In a variant embodiment the tube is formed in two parts coupled by a bellows. Each end of the bellows is fastened to the corresponding tube by sealing in accordance with well-known techniques. Here the elements which are to be assembled i.e. the two half tubes exhibit like coefficients of thermal expansion, such giving rise to a type of weld which is the same for each end of the bellows. The purpose of the bellows in this particular citation is to obtain a precise alignment of the two half tubes, one following the other in a manner to obtain a perfect alignment of the axial optical path. The purpose of the present invention is completely different since the bellows as proposed provides a sealed assembly of two elements having different coefficients of thermal expansion in order to enable the displacement of such elements relative to one another following temperature differences which can arise.
The gas laser of Japanese patent document No. JP-A-60 28281 shows that the end mirrors are fastened to the discharge tube by means of bellows according to a known technique in order to maintain the distance between the mirrors constant when the temperature varies. Nothing however is known of the manner of sealing such bellows to the parts which they connect.
The Swiss patent document No. 474.880 describes an arrangement for pumping crystal lasers. There will be seen in the single figure of this document bar 1 which is supported by flanges 22 and 23 including structures 20. The sealing of bar 1 with respect to the cooling fluid filling space 15 is assured on each side by a toroidal seal 21.
Likewise there are known certain gas lasers using CO.sub.2 the discharge tube of which, made of ceramic, is supported with play by the anode and cathode chambers, the seal being obtained by means of O-rings formed from polymers.
The last two cited examples show that it is customary to employ toroidal seals generally of plastic material when the elements employed do not provide the same coefficient of thermal expansion, as is the case when one is concerned with assembling a metallic element to a non-metallic element. However, experience shows that these seals lack reliability. Effectively, since they are subjected to severe stresses under normal use: important temperature variations, plasma bombardment, they age rapidly and must be replaced many times. They thus necessitate frequent maintenance shut-downs which bring about loss of time and high costs. It must likewise be mentioned that such seals are not absolutely impermeable and that for a gas laser employing them, it will be periodically necessary to re-establish at the interior of the cavity a pressure suitable to assure proper operation of the arrangement.